Re: [PATCH v2 27/28] mm: memcontrol: eliminate the problem of dying memory cgroup for LRU folios

[Qi Zheng <qi.zheng@linux.dev>]

On 12/18/25 10:06 PM, Johannes Weiner wrote:
> On Wed, Dec 17, 2025 at 03:27:51PM +0800, Qi Zheng wrote:
>> From: Muchun Song <songmuchun@bytedance.com>
>>
>> Pagecache pages are charged at allocation time and hold a reference
>> to the original memory cgroup until reclaimed. Depending on memory
>> pressure, page sharing patterns between different cgroups and cgroup
>> creation/destruction rates, many dying memory cgroups can be pinned
>> by pagecache pages, reducing page reclaim efficiency and wasting
>> memory. Converting LRU folios and most other raw memory cgroup pins
>> to the object cgroup direction can fix this long-living problem.
> 
> This is already in the coverletter. Please describe here what the
> patch itself does. IOW, now that everything is set up, switch
> folio->memcg_data pointers to objcgs, update the accessors, and
> execute reparenting on cgroup death.

Got it, will do.

> 
>> Finally, folio->memcg_data of LRU folios and kmem folios will always
>> point to an object cgroup pointer. The folio->memcg_data of slab
>> folios will point to an vector of object cgroups.
> 
>> @@ -223,22 +223,55 @@ static inline void __memcg_reparent_objcgs(struct mem_cgroup *src,
>>   
>>   static inline void reparent_locks(struct mem_cgroup *src, struct mem_cgroup *dst)
>>   {
>> +	int nid, nest = 0;
>> +
>>   	spin_lock_irq(&objcg_lock);
>> +	for_each_node(nid) {
>> +		spin_lock_nested(&mem_cgroup_lruvec(src,
>> +				 NODE_DATA(nid))->lru_lock, nest++);
>> +		spin_lock_nested(&mem_cgroup_lruvec(dst,
>> +				 NODE_DATA(nid))->lru_lock, nest++);
>> +	}
>>   }
> 
> Looks okay to me. If this should turn out to be a scalability problem
> in practice, we can make objcgs per-node, and then reparent lru/objcg
> pairs on a per-node basis without nesting locks.
> 
>>   static inline void reparent_unlocks(struct mem_cgroup *src, struct mem_cgroup *dst)
>>   {
>> +	int nid;
>> +
>> +	for_each_node(nid) {
>> +		spin_unlock(&mem_cgroup_lruvec(dst, NODE_DATA(nid))->lru_lock);
>> +		spin_unlock(&mem_cgroup_lruvec(src, NODE_DATA(nid))->lru_lock);
>> +	}
>>   	spin_unlock_irq(&objcg_lock);
>>   }
>>   
>> +static void memcg_reparent_lru_folios(struct mem_cgroup *src,
>> +				      struct mem_cgroup *dst)
>> +{
>> +	if (lru_gen_enabled())
>> +		lru_gen_reparent_memcg(src, dst);
>> +	else
>> +		lru_reparent_memcg(src, dst);
>> +}
>> +
>>   static void memcg_reparent_objcgs(struct mem_cgroup *src)
>>   {
>>   	struct obj_cgroup *objcg = rcu_dereference_protected(src->objcg, true);
>>   	struct mem_cgroup *dst = parent_mem_cgroup(src);
>>   
>> +retry:
>> +	if (lru_gen_enabled())
>> +		max_lru_gen_memcg(dst);
>> +
>>   	reparent_locks(src, dst);
>> +	if (lru_gen_enabled() && !recheck_lru_gen_max_memcg(dst)) {
>> +		reparent_unlocks(src, dst);
>> +		cond_resched();
>> +		goto retry;
>> +	}
>>   
>>   	__memcg_reparent_objcgs(src, dst);
>> +	memcg_reparent_lru_folios(src, dst);
> 
> Please inline memcg_reparent_lru_folios() here, to keep the lru vs
> lrugen switching as "flat" as possible:
> 
> 	if (lru_gen_enabled()) {
> 		if (!recheck_lru_gen_max_memcgs(parent)) {
> 			reparent_unlocks(memcg, parent);
> 			cond_resched();
> 			goto retry;
> 		}
> 		lru_gen_reparent_memcg(memcg, parent);
> 	} else {
> 		lru_reparent_memcg(memcg, parent);
> 	}

Looks better, will change to this style.

> 
>> @@ -989,6 +1022,8 @@ struct mem_cgroup *get_mem_cgroup_from_current(void)
>>   /**
>>    * get_mem_cgroup_from_folio - Obtain a reference on a given folio's memcg.
>>    * @folio: folio from which memcg should be extracted.
>> + *
>> + * The folio and objcg or memcg binding rules can refer to folio_memcg().
> 
>        See folio_memcg() for folio->objcg/memcg binding rules.

OK, will do.